Method for forming radiation images

Abstract

A tube voltage, at which each of at least two kinds of radiation having different energy levels is produced, is adjusted such that a ratio between a change in the image density of bones, which change occurs due to a beam hardening phenomenon of the radiation having a high energy level, and a change in the image density of the bones, which change occurs due to a beam hardening phenomenon of the radiation having a low energy level, may become approximately equal to a ratio between subtraction factors employed in a subtraction process. Each of at least two kinds of the radiation, each of which has been produced at the thus adjusted tube voltage, is irradiated to one of at least two stimulable phosphor sheets, and radiation images of the object are thereby stored on the stimulable phosphor sheets. The radiation images are then photoelectrically read out, and digital image signals are thereby obtained. Image signal components of the digital image signals, which represent corresponding picture elements in the radiation images, are then subtracted from each other, and a difference signal is thereby obtained which represents the image of only the bones represented by the radiation images.

What is claimed is: 
     
       1. A method for forming a radiation image, comprising the steps of: irradiating an object comprising bone and soft tissue with first and second radiation having first and second energy levels, respectively, while exposing first and second stimulable phosphor sheets to said first and second radiation, respectively, to cause said first and second stimulable phosphor sheets to store first and second radiation images, respectively, of said bone and soft tissue of said object;   exposing said first and second stimulable phosphor sheets to stimulating rays to cause said first and second stimulable phosphor sheets to emit first and second light, respectively, based on said first and second radiation images;   detecting and converting said first and second light into first and second image signals, respectively; and   subtracting said first and second image signals from each other to provide a difference signal representing an image of said bone of said object;   said irradiating step comprising the step of: adjusting first and second tube voltages to be first and second optimum tube voltages which generate said first and second radiation having optimum energy levels so that when said first and second image signals are subtracted from each other in said subtracting step, a portion of said first image signal representing a change in density of said first radiation image of said bone occurring due to a beam hardening phenomenon of said first radiation and a portion of said second image signal representing a change in density of said second radiation image of said bone occurring due to a beam hardening phenomenon of said second radiation cancel each other.     
     
     
       2. A method for forming a radiation image as claimed in claim 1, wherein said first and second radiation are each X-rays. 
     
     
       3. A method for forming a radiation image as claimed in claim 1, wherein each of the stimulable phosphor sheets is two-dimensionally scanned with the stimulating rays. 
     
     
       4. A method for forming a radiation image as claimed in claim 1, wherein the stimulating rays are a laser beam. 
     
     
       5. A method as claimed in claim 1, wherein said first and second image signals are digital signals. 
     
     
       6. A method as claimed in claim 1, wherein said first energy level is higher than said second energy level. 
     
     
       7. A method for quantitatively analyzing bone calcium, comprising the steps of: irradiating an object comprising bone and soft tissue with first and second radiation having first and second energy levels, respectively, while exposing first and second stimulable phosphor sheets to said first and second radiation, respectively, to cause said first and second stimulable phosphor sheets to store first and second radiation images, respectively, of said bone and soft tissue of said object;   exposing said first and second stimulable phosphor sheets to stimulating rays to cause said first and second stimulable phosphor sheets to emit first and second light, respectively, based on said first and second radiation images;   detecting and converting said first and second light into first and second image signals, respectively;   subtracting said first and second image signals from each other to provide a difference signal representing an image of said bone of said object; and   quantitatively analyzing calcium in said bone based on said difference signal;   said irradiating step comprising the step of: adjusting first and second tube voltages to be first and second optimum tube voltages which generate said first and second radiation having optimum energy levels so that when said first and second image signals are subtracted from each other in said subtracting step, a portion of said first image signal representing a change in density of said first radiation image of said bone occurring due to a beam hardening phenomenon of said first radiation and a portion of said second image signal representing a change in density of said second radiation image of said bone occurring due to a beam hardening phenomenon of said second radiation cancel each other.     
     
     
       8. A method for quantitatively analyzing bone calcium as claimed in claim 7, wherein said first and second radiation are each X-rays. 
     
     
       9. A method for quantitatively analyzing bone calcium as claimed in claim 7, wherein each of the stimulable phosphor sheets is two-dimensionally scanned with the stimulating rays. 
     
     
       10. A method for quantitatively analyzing bone calcium as claimed in claim 7, wherein the stimulating rays are a laser beam. 
     
     
       11. A method as claimed in claim 7, wherein: said irradiating step irradiates a pattern of bone calcium reference material with said first and second radiation, respectively, while exposing first and second stimulable phosphor sheets to said first and second radiation, respectively, to cause each of said first and second radiation images to include a radiation image of said bone calcium reference material;   said difference signal includes a first portion representing an image of said bone of said object and a second portion representing an image of said bone calcium reference material; and   said quantitatively analyzing step comprises the steps of: generating said image of said bone of said object represented by said first portion of said difference signal and said image of said bone calcium reference material represented by said second portion of said difference signal; and   comparing said generated image of said bone of said object and said generated image of said bone calcium reference material with each other.     
     
     
       12. A method as claimed in claim 7, wherein said first and second image signals are digital signals. 
     
     
       13. A method as claimed in claim 7, wherein said first energy level is higher than said second energy level. 
     
     
       14. A method for quantitatively analyzing bone calcium, comprising the steps of: irradiating first and second stimulable phosphor sheets with first and second radiation having first and second energy levels, respectively, to cause said first and second stimulable phosphor sheets to store first and second radiation energy, respectively;   exposing said first and second stimulable phosphor sheets to stimulating rays to cause said first and second stimulable phosphor sheets to emit first and second energy light, respectively, based on said first and second radiation energy;   detecting and converting said first and second energy light into first and second energy signals, respectively;   irradiating an object comprising bone and soft tissue with said first and second radiation, respectively, while exposing said first and second stimulable phosphor sheets to said first and second radiation, respectively, to cause said first and second stimulable phosphor sheets to store first and second radiation images, respectively, of said bone and soft tissue of said object;   exposing said first and second stimulable phosphor sheets to stimulating rays to cause said first and second stimulable phosphor sheets to emit first and second light, respectively, based on said first and second radiation images;   detecting and converting said first and second light into first and second image signals, respectively;   subtracting said first energy signal and said first image signal from each other, and said second energy signal and said second image signal from each other, to provide first and second subtracted signals, respectively;   subtracting said first and second subtracted signals from each other to provide a difference signal representing an image of said bone of said object; and   quantitatively analyzing calcium in said bone based on said difference signal;   said object irradiating step comprising the step of: adjusting first and second tube voltages to be first and second optimum tube voltages which generate said first and second radiation having optimum energy levels so that when said first and second subtracted signals are subtracted from each other in said subtracted signals subtracting step, a portion of said first subtracted signal representing a change in density of said first radiation image of said bone occurring due to a beam hardening phenomenon of said first radiation and a portion of said second subtracted signal representing a change in density of said second radiation image of said bone occurring due to a beam hardening phenomenon of said second radiation cancel each other.     
     
     
       15. A method as claimed in claim 14, wherein said first and second image signals and said first and second energy signals are digital signals. 
     
     
       16. A method as claimed in claim 14, wherein said first energy level is higher than said second energy level.